For an apparatus to measure the tension on a long steel member such as the cable of a hanging structure, the tension member of a ground anchor and the wire rope of transport equipment, a load cell (strain gauge type, differential transformer type) having a hole at the center through which a steel member is passed, or a hydraulic pressure disk is often used. These apparatuses are used, located between an anchorage fastened to a chuck with the object to be measured passed through and a fixture attached to an end of the target object. Therefore, the measurement apparatus must be set at the time of installing the object that will be subsequently subjected to measurement. The conventional measurement apparatuses could not be applied to measuring the tension of a member that was already installed.
By employing the stress measurement method utilizing the stress magnetic effect appearing at a magnetic element such as of steel (the phenomenon of variation in magnetization by stress), the measurement apparatus can be installed at an arbitrary position of a long steel member that has already been installed to carry out tension measurement. One known stress measurement method utilizes the variation in the magnetic permeability of a magnetic element caused by stress. The target object is magnetized up to the range of approach to saturation magnetization (the region where the hysteresis loop of magnetization property is closed; “rotating magnetization region” in terms of magnetic physics), and an alternating magnetic field of small amplitude is applied. The amplitude of the magnetic flux density with respect to the alternating magnetic field is measured to obtain the magnetic permeability for evaluating the stress (refer to Patent Document 1). Another approach is proposed utilizing the phenomenon that the magnetic flux flowing towards space varies when the magnetic flux passing through the interior of a magnetic element changes by the stress. The stress is evaluated by measuring the density of the magnetic flux leaking out from the permanent magnet that magnetizes the measuring object to the space located opposite of the measuring object (refer to Patent Document 2).